In a previous study the development and decline of the progestational endometrium of the rabbit were described and divided into clearly defined phases with distinctive cellular events. It is proposed to reexamine each of these phases in terms of their hormonal basis (progesterone, 20 alpha and estradiol) and to clarify special problems. Hormonal analysis will be by substitution of these steroidal hormones after bilateral ovariectomy. Special problems will be analysed as follows: (1) the significance of the multinucleated cells, their derivation from uninucleated cells and their possible use as indicators of progesterone levels (number of nuclei per cytoplasmic envelope), (2) the significance of temporal and cytological differences between the antimesometrial and mesometrial areas of the uterus in relation to the determination of the definitive implantation site, (3) the mingling and/or fusion of maternal and trophoblastic nuclei at the implantation site, (4) the manner of development and significance of ciliation of the endometrial epithelium as a reflection of declining progesterone or dominance of 20 alpha, (5) the induction of reovulation within an existing progestational cycle and a detailed study of the involution of the endometrial epithelium and the initiation of a new progestational cycle, (6) studies on asynchrony between blastocyst and endometrium with critical reference to the phases and the detailed light and electron microscopy of the interacting surfaces, (7) the attempt to induce delayed implantation by hypohysectomy of the early pregnant animal, (8) a search for a proteolysin or zonolysin as a specific secretory event related to known cytological facts and as possible prerequisite for implantation. Vascular studies will also be made in special relation to implantation.